One European official summed up the challenge succinctly: how do you ensure a column of tanks stationed in Spain "doesn't reach Poland after the war"?

For now, that is far from guaranteed.

Currently, tanks and other heavy equipment still require country-by-country authorisations to transit through EU territory. Even with permits in hand, convoys often have to take lengthy detours to avoid roads or bridges too weak to bear their weight.

"Speed is the cornerstone of warfare and deterrence," EU defence chief Andrius Kubilius told reporters ahead of Wednesday's presentation of a European Commission plan to address the challenge.

With Europe racing to build its defences since Russia's 2022 invasion of Ukraine, EU auditors warned earlier this year that moving troops and armaments across the 27-nation bloc remained "problematic" and that it was unclear "who does what".

The European Court of Auditors called governance arrangements for military mobility in the EU "complex and fragmented" -- noting for instance that tanks from one country cannot move through another if they are heavier than road traffic regulations allow.

One telling example: France encountered difficulties in 2022 transporting tanks to Romania after Germany said the heavy equipment could not travel by road, forcing officials to charter trains.

Around 500 key choke points have been identified along potential corridors for troop movements through Europe in the event of war, as part of the commission's plan.

Most need urgent upgrades to bring about the kind of mobility that has become a priority since Russia's invasion of Ukraine.

Underscoring the need to act, Kubilius quoted the commander of US forces in Europe during World War I, General John J. Pershing, who famously declared: "Infantry wins battles; logistics wins wars."

- Stress tests -

To tackle bottlenecks, the commission is proposing a single permit valid across the EU, replacing the current patchwork of authorisations -- some of which must be requested 45 days in advance.

In cases of emergency, Brussels also wants clear rules for priority passage to avoid delays.

A so-called solidarity mechanism pooling national dual-use defence resources such as trucks for rapid access is part of the plan.

Kubilius said the EU plans marked a "substantial step forward."

To make sure the system works, the commission plans regular "stress tests" to check whether infrastructure can handle heavy military traffic.

Brussels wants to earmark 17 billion euros (nearly $20 billion) between 2028 and 2034 to boosting military mobility -- 10 times more than under its previous long-term budget.

Awareness of the problem is not new: the commission has already launched two action plans, the latest in November 2022, which got a lacklustre reception from the Court of Auditors.

In its defence, Brussels argues military mobility is a relatively new policy area, where EU institutions have limited agency given member states' control over defence.

The commission also on Wednesday unveiled a plan to bring Europe's defence industry closer to emerging technologies -- from artificial intelligence to quantum computing and space systems.

While arms manufacturers already use these tools extensively, Brussels wants to encourage joint projects and is pushing in particular for the bloc's emerging "AI factories" aimed at testing artificial intelligence solutions to be opened to defence firms.